udf: Fix extension of the last extent in the file

[ Upstream commit 83c7423d1e ]

When extending the last extent in the file within the last block, we
wrongly computed the length of the last extent. This is mostly a
cosmetical problem since the extent does not contain any data and the
length will be fixed up by following operations but still.

Fixes: 1f3868f068 ("udf: Fix extending file within last block")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/udf/inode.c

@@ -599,7 +599,7 @@ static void udf_do_extend_final_block(struct inode *inode,
 	 */
 	if (new_elen <= (last_ext->extLength & UDF_EXTENT_LENGTH_MASK))
 		return;
-	added_bytes = (last_ext->extLength & UDF_EXTENT_LENGTH_MASK) - new_elen;
+	added_bytes = new_elen - (last_ext->extLength & UDF_EXTENT_LENGTH_MASK);
 	last_ext->extLength += added_bytes;
 	UDF_I(inode)->i_lenExtents += added_bytes;